Methods and apparatus for ion cyclotron resonance spectroscopy are known, for example, from an article by G. Perisod and T. Gaumann in Chimia 34, 271 (1980) and from German published patent application OS No. 25 46 225. The first named publication describes a method in which the frequency of the high frequency measuring field is continuously varied to determine the cyclotron resonance frequencies. In the method disclosed in the second publication, a wide band excitation of the sample substance is performed, followed by a Fourier transformation analysis of the cyclotron resonance signals obtained from the excitation. The HF signals required for the frequency scanning method according to the first publication may be obtained, for example, by means of a tunable frequency synthesizer, while the wide band excitation signal for the method according to the second publication may be obtained by an appropriate modulation of a HF signal. In both techniques, an improvement in the signal-to-noise ratio may be obtained by repeating the measurement several times and then adding up the produced signals, so that also weak lines can be detected and recorded.
From U.S. Pat. No. 3,535,512 and No. 3,502,867 it is also known to employ a high frequency measuring field having two discrete frequencies which correspond to the cyclotron resonance frequencies of two different species of ions in order to excite these two different kinds of ions to be able to study the interactions occurring between them.
The prior art methods are afflicted with problems which arise from the limited dynamics of a spectrometer. This limited dynamics of a spectrometer is due to the fact that the ion density in the measuring cell must be limited to values at which no adverse space charge effects will occur yet. Since the ionization of a sample substance is usually not selective, the sample substance may include ions which produce very strong lines, but are not of much interest to the particular analysis to be undertaken. This is the case, for instance, where the dominating lines stem from a vehicle gas or a solvent for the substance under study. Moreover, the maximal dynamics of the receiver system of the spectrometer sets further limits as to the highest possible ratio between the strongest line and the weakest line.